The technical field of this invention is power assist in vehicle braking systems.
Recent attention of brake system designers has been given to the design of brake system controls that will assist a vehicle operator in maximizing the useful application of braking force in an emergency or panic situation to obtain vehicle deceleration to a stop or lower speed in the shortest time and/or distance. In such a situation, wherein it is desired to stop the vehicle in the shortest time and/or distance possible, a high intensity braking effort is required. The phrase xe2x80x9chigh intensityxe2x80x9d is used here to denote a braking force application that is both fast and of great magnitude.
A suggested method of providing such assist is to provide apparatus which, when it detects an operator request for high intensity braking through dynamic movement of the brake pedal, provides an immediate, maximal increase in braking pressure to trigger the vehicle""s anti-lock braking control and then to maintain the pressure sufficiently high to allow the antilock braking control to control the deceleration at the highest rate obtainable with the surface coefficient of friction of the road. The increase in braking pressure may be obtained by the control taking over the source of braking assist pressure from control of the vehicle operator, once the desire for such maximal braking is detected, until the vehicle operator signals the end of the situation or the vehicle is stopped. The method overcomes the tendency of many vehicle operators in an emergency to back off somewhat on the brake pedal after an initial hard activation, as illustrated in curve 150 of the graph of FIG. 4, which represents the master cylinder pressure through a braking event that begins with high intensity but undergoes a sag as the driver backs off the braking force somewhat before increasing it again. This tendency results in under-utilization of the maximum braking power obtainable in modern antilock braking systems.
Although some brake system designers have suggested controls working through vacuum assist units such as are commonly used in power braking systems, it is also possible to provide brake assist with a hydraulic assist system incorporated in the anti-lock brake modulator. Such a system provides an isolation valve between the master cylinder and the brake apply mechanism and a hydraulic pump for brake fluid which is activated by the control to quickly provide a hydraulic supply pressure much higher than master cylinder pressure to the anti-lock apply valves controlling the braking pressure at the individual wheel brake apply units. The pressure generated by the pump is typically limited by a pressure relief valve, which is provided a predetermined blow-off value equal to the highest hydraulic pressure contemplated or allowed. When this system is adapted for emergency braking control, this may result in supply pressures when the pump is in use that are significantly higher than required for most functions involving the anti-lock module, so that the difference between the supply pressure and the pressure provided to the wheel brake units is very large. The result of this is excess noise, from the pump and from hydraulic hammering as valves open and close, and also excess energy use and wear as the pump provides pressures significantly higher than those required for most braking operations using the modulator.
The control of this invention controls a vehicle hydraulic braking supply pressure in a vehicle braking system having a wheel brake, an apply valve for applying the supply pressure to the wheel brake when opened and isolating the wheel brake from the supply pressure when closed, a brake pedal, a master cylinder responsive to the brake pedal to generate a master cylinder pressure and a motor driven hydraulic pump having an outlet to the apply valve. Hydraulic valve apparatus connected between the master cylinder and the apply valve has a first condition providing open hydraulic pressure communication from the master cylinder to the apply valve and a second condition preventing hydraulic pressure communication from the master cylinder to the apply valve. The hydraulic valve apparatus is provided with an electrically controllable pressure release valve permitting controlled pressure relief from the pump to the master cylinder. A sensor indicating operator requested braking intensity is monitored to detect an operator intended high braking intensity event.
Upon detection of the operator intended high braking intensity event, the control places the hydraulic valve means in its second condition, operates the pump and controls the electrically controllable pressure relief valve to generate a maximum predetermined supply pressure to the apply valve for an initial predetermined time The control senses vehicle deceleration and, after the initial predetermined time, controls the electrically controllable pressure relief valve responsive to the sensed vehicle deceleration so as to decrease the supply pressure provided to the apply valve to a level just sufficient to provide the sensed vehicle deceleration.
In a preferred embodiment of this invention, the vehicle braking system has an anti-lock brake control responsive to wheel spin to control the apply valve to modulate the supply pressure so as to reduce wheel brake pressure and minimize the wheel spin and the maximum predetermined supply pressure is calibrated to produce wheel spin and thus activate the anti-lock brake control.
A preferred embodiment of this invention periodically derives a desired target wheel pressure varying with the magnitude of the sensed vehicle deceleration, derives a target wheel pressure error between a present target wheel pressure and the desired target wheel pressure, slew rate limits the target wheel pressure error, decreases the target wheel pressure by the slew rate limited target wheel pressure error controls the electrically controllable pressure relief valve to reduce the supply pressure toward the decreased target wheel pressure.
In a preferred embodiment of this invention, after the initial predetermined time the motor speed of the pump is controlled in response to the target wheel pressure error so as to maintain pump output pressure just higher than the supply pressure maintained by the electrically controllable pressure relief valve.